1. Field of the Invention
The present invention concerns device and method for heat-insulating at least one sub-marine pipe situated at a great depth.
More particularly, it concerns the pipes connecting the bottom of the sea to anchored installations floating on the surface.
The technical sector of the invention is the field concerning the production and mounting of insulation systems outside and around pipes in which hot effluent circulates for which it is desired to limit heat losses.
2. Disscusion of the Related Art
This invention is more particularly applicable to the developments of deep sea petroleum fields, that is petroleum installations installed in the sea in which the surface equipment is generally situated on floating structures, the well heads being at the bottom of the sea. The pipes concerned by the present invention more particularly are risers or surface/bottom linking pipes rising to the surface, but also pipes connecting the well heads to said surface/bottom linking pipes.
Deep sea developments are carried out by water depths currently reaching 1500 m. Future developments are envisaged by water depths up to 3000–4000 m and beyond.
The main application of the invention concerns the heat insulation of sub-marine or sub-aquatic immersed pipes or ducts and situated more particularly at a depth of more than 300 meters and carrying hot petroleum products whose excessive cooling would cause problems during normal production and when production is stopped.
In fact, in these types of applications, many problems occur if the temperature of the petroleum products reduces by a significant value with respect to their production temperature which is often more than 60 to 80° C. when the temperature of the surrounding water at a great depth may be basically lower than 10° C. and reach 4° C. If the petroleum products cool below 30–60° C. for an initial temperature of between 70 and 80° C., the following is generally observed:                a high increase of viscosity which then reduces the flow of the pipe,        a precipitation of dissolved paraffin which then increases the viscosity of the product and whose deposit can reduce the internal effective diameter of the pipe,        the flocculation of asphaltenes bringing about the same problems,        the sudden, compact and massive formation of gas hydrates which precipitate at high pressure and at a low temperature, thus suddenly blocking off the pipe.        
Paraffins and asphaltenes remain stuck to the wall and then need to be cleaned by scraping the inside of the pipe. On the other hand, the hydrates are even more difficult and sometimes impossible to resorb.
The aim of the heat insulation of these pipes is therefore to delay cooling of the petroleum effluent carried, not only during established production mode so that their temperature is for example at least 40° C. when arriving on the surface for a production temperature at the inlet of the pipe of between 70 and 80° C., but also in cases of a reduction or even stoppage of production so as to ensure that the temperature of the effluent does not go below for example 30° C. so as to limit the problems mentioned above or at least enable them to be rendered reversible.
In the case of the installation of single pipes or bundles of pipes, said pipes are generally preferably prefabricated on shore in unit lengths of between 250 and 500 m which are then drawn from the open sea with the aid of a tow boat. In the case of a tower type bottom/surface link, the pipe length generally represents 50 to 95% of the water height, that is it can reach 2400 m for a water depth of 2500 m. When produced on shore, the first unit length is pulled from the sea and joined end-to-end to the next one, the tow boat keeping the unit in traction during the adding-on phase which may last several hours or even days. When the whole pipe or bundle of pipes has been placed in the water, the unit is pulled up to the site, generally on the subsurface approximately horizontal where it is then “canted”, that is tilted into a vertical position, so as to reach the vertical position and then being placed into its final position.
A known insulation device exists having at least one sub-marine pipe (which may in fact be a single pipe or assembled with other pipes thus constituting bundles) to be placed on the bottom at a great depth and comprising an insulation external covering surrounding it and a protective envelope.
The insulation of the pipe(s) or pipe bundle commonly known as “bundles” is/are then protected by an external envelope having a double function:                firstly avoiding damage which may occur when producing or towing as in placing, especially in shallow water zones, said towing able in certain cases to be effected over distances ranging up to several hundreds of kilometers. To this effect, relatively resistant materials are used, such as steel, thermoplastic or duroplastic compounds or even a composite material;        secondly creating a sealed containment around the insulation system. This containment is required for external insulation coverings constituted by materials subject to migration including fluid compounds.        
In effect, through sea bottoms of 2000 m, the hydrostatic pressure is about 200 bars, namely 20 megapascals, which requires that all the pipes and their insulation coverings need to be able to resist, not only these pressures without deteriorating during pressurisations and depressurisations of the pipe in which the hot fluid circulates, but also temperature cycles which generate volume variations of the various components and thus of positive or negative pressures possibly resulting in the partial or total destruction of the envelope, either by exceeding the admissible stresses or by this external envelope imploding (negative internal pressure variations).
The patents FR99/00985 and WO 00/40886 describe a method and device concerning a solid/liquid phase change and melting latent heat insulation material able to restore calories in the internal pipe and confined inside a sealed ductile envelope which makes it able to follow the expansion and contraction of the various components under the influence of all the environment parameters, including the internal and external temperatures. Thus, the pipe is confined inside a flexible thermoplastic envelope, possibly circular, made in particular of polyethylene or polypropylene, the increase or reduction of the internal volume due to the temperature variations and comparable to respiration being absorbed by the flexibility of the envelope constituted, for example, by a thermoplastic material having a large elastic limit. So as to resist the mechanical stresses, a semi-rigid envelope is preferably used constituted by a resistant material such as steel or a composite material, such as a compound embodied from a binder such as an epoxy resin and mineral or organic fibres such as glass or carbon fibres, but the bundle here is given an ovoid or flattened shape with or without any counter curve which on a constant perimeter provides it with a section smaller than the corresponding circle. Thus, the “respiration” of the bundle shall result, in the case of an increase and reduction of the volume in respectively a “recircling” of the envelope or an accentuation of flattening of the envelope. In this case, the bundle/envelope unit is denoted by the term “flat bundle”, as opposed to a circular envelope.
There is also the method used on the GANET field which consists of prefabricating onshore a sealed circular bundle having no insulating complex but filled with an inert gas and then of towing it onto the site and installing it at great depth so as to finally fill it with a mono-ethylene glycol-based insulating compound added with viscous-rendering agents. So as to absorb the volume variations created by the temperature variations without creating unacceptable stresses inside the circular steel envelope, a ductile pipe pressurised with nitrogen has been installed inside the bundle along the latter.
These prior embodiments have been described for applications in which the pipe rests horizontally on the bottom of the sea
In the case of a bottom/surface link, for example the vertical portion of a tower or even the small chain section connecting the top of the tower to the surface support or even pipes resting on a steep slope of the bottom of the sea, the external pressure varies along the pipe and gradually decreases when it is brought back up to the surface. In the case of fluid or pasty insulating materials, when this bottom has a density less than that of the water of the sea, generally a density of between 0.8 and 0.85, the differential pressure between the outside and the inside shall vary along said pipe and gradually increasing when it is lifted up to the surface. Thus, this results in deformations accentuated in those portions exhibiting the differential pressure maximum, thus inducing significant transfers of fluid parallel to the longitudinal axis of said pipe. In addition, the transfers are amplified by “breathing” phenomena due to the temperature variations described above.
A “flat bundle” is sensitive to the pressure variations due to the slopes: excess pressure at the bottom, depression at the top, and the pulling phase is critical as the length, possibly reaching several kilometers, the bundle is never made to be perfectly horizontal and this results in significant differential pressure variations during towing and particularly during the canting operation.
When the bundle is in the vertical position or at the bottom of the sea on a steep slope, the pressure differential created by the low density of the insulating material associated with the volume variation created by the heat expansion of the insulation material generates movements of the insulating material the external envelope needs to be able to support. Thus, efforts are sought to avoid the movements of particles parallel to the axis of the bundle, that is migrations of the insulation material between two distant zones of the bundle as they risk destroying the actual structure of the insulating material.
So that the bundle behaves properly throughout its stay at sea, it is desirable that it does not comprise any residual gas. In fact, in the case of a semi-fluid or pasty insulating complex, any pocket of gas resulting from the production process shall have repercussions, firstly concerning transport as, the moment the bundle is moved to a significant depth, the ambient pressure compresses the residual gas which risks significantly reducing buoyancy, this possibly leading to dangerous situations, not only for the materials but also for personnel; and secondly, during vertical positioning, all the compressed gas pockets move to the top of the bundle, thus risking creating a significant pipe length without any insulating component.